Field of the Invention
This invention relates in general to a display device for showing video information whose number of scanning lines changes over elapsed time per each field and relates in particular to a display device ideal for preventing vertical jitter occurring during the display of video information having changes in the number of scanning line that occur at random. This invention is suited for display for instance of digital broadcasts or joint display of both digital broadcasts and analog broadcasts.
Digital technology has been making steady progress in the area of broadcasting in recent years. In order for display equipment such as for instance television receivers to keep pace with this progress, the display equipment must be able to display digital broadcast signals as well as analog broadcast singals and VTR playback signals. Circuit systems are therefore being designed to make receivers compatible with digital broadcasts. A decoded digital broadcast signal is therefore written temporarily into a field memory and then utilized in a read-out system. Analog signals of the currently used NTSC method are subjected to analog/digital (A/D) conversion and then input to a digital signal processing circuit and written along with the digital broadcast signal into a field memory.
The writing and reading of video information into the field memory is performed by utilizing the read block and vertical and horizontal synchronizing signals. However, when the digital broadcast signal is for instance an MPEG2 type and has been signal-compressed, no synchronizing signal is sent from the transmitter. Accordingly, a synchronizing signal must be generated and a read signal formed by the receiver in order to read out the information from the field memory. However a difference in frequency sometimes occurs between the write signal for writing into the field memory, and the read signal for readout from the field memory. This difference in frequency results in reversed timing or an opposite phenomenon called "skip" occurring between the memory writing and readout operations.
When this memory skip occurs, the current field image on one frame of the screen and the image from one previous field are mixed together thus requiring some contrivance to prevent memory skip from occurring. The changes particularly in the vertical frequency are particularly large in cases such as the custom playback of VTR signals. One method to prevent memory skip from occurring at such times is by changing the number of scanning lines per field. One example of this is television receivers in Europe capable of receiving broadcast teletext or subscript transmissions. These receivers are set to have alternate scanning lines of 312 lines by 313 lines per field (PAL etc.) or 262 lines by 263 lines (NTSC) with noninterlaced scanning used for display of the teletext broadcast screen.
However, a feedback circuit for uniform vertical amplitude is provided in the vertical deflection circuit of the CRT display. Consequently, a vertical jitter at 60 Hz occurs when attempting to display video information in which the alternate scanning lines are 312 lines by 313 lines (or 262 by 263 lines) per field on the CRT display. A technology has been proposed to reduce this vertical jitter which is known in Japanese Examined Utility Model Publication No. 7-44130. This method isolates the circuit for aligning vertical amplitude and the feedback circuit, and utilizes a differentiator circuit consisting of a resistor and capacitor to cut the DC components and apply feedback signals to the vertical amplifier amplitude control terminal.